R3303-HP HSR6800 Routers Layer 2 - WAN Configuration Guide
47
RTP is a UDP protocol using fixed port number and format. An RTP packet comprises a 40-byte header
and a data section. The 40-byte header, which contains a 20-byte IP header, an 8-byte UDP header, and
a 12-byte RTP header, is large compared with the payload, which is usually 20 bytes to 160 bytes. To
reduce bandwidth consumption, use IPHC to compress RTP packet headers. After compression, the
40-byte header can be reduced to 2 to 5 bytes. If the payload is 40 bytes, the compression ratio will be
(40+40) / (40+5), about 1.78, which is very efficient.
To configure IP header compression:
Ste
p
Command
Remarks
1. Enter system view.
system-view N/A
2. Enter interface view.
interface interface-type
interface-number
N/A
3. Enable IP header compression.
ppp compression iphc
[ nonstandard ]
By default, IP header compression
is disabled.
4. Set the maximum number of
connections for which an interface
can perform RTP header
compression.
ppp compression iphc
tcp-connections number
Optional.
The default setting is 16.
5. Set the maximum number of
connections for which an interface
can perform TCP header
compression.
ppp compression iphc
rtp-connections number
Optional.
The default setting is 16.
Configuring LFI
The router supports Link Fragmentation and Interleaving (LFI) for improving transmission efficiency on PPP
links.
CAUTION:
Disabling LFI also removes the user-configured settings of maximum LFI fragment delay and size.
On a low-speed serial link, packets of real-time interactive communications (such as Telnet and VoIP)
might be blocked or delayed if packets of other applications are also transmitted across the link. For
example, if a voice packet arrives when large packets are being scheduled and waiting to be transmitted,
it must wait until all the large packets have been transmitted. For the real-time applications, such as VoIP,
delays longer than 100 or 150 ms cause voice quality to drop dramatically and cannot be tolerated.
On a 56 kbps link, it costs approximately 215 ms to transmit a 1500-byte packet (the size of the MTU of
common links). To confine the delay of transmitting time-sensitive packets on low-speed links (such as 56
kbps frame relay channels or 64 kbps ISDN B channels) to an acceptable level, a method is required to
fragment larger packets and adding both the smaller packets and fragments of the large packet to an
output queue.
LFI reduces delays and jitters on low-speed links by fragmenting large packets into small fragments and
transmitting them along with small packets. The fragmented datagrams are reassembled at the
destination.
Figure 11 illu
strates the process of LFI. When large packets and small voice packets arrive at an interface
that is enabled with WFQ at the same time, the large packets are fragmented into small fragments, which
are then added to the queues along with the voice packets.